Nano-particles are currently receiving significant interest as fillers in rubber compositions. Such combinations often provide dimensions and physical properties that differ from those of the bulk material. The use of nano-particles as fillers is thought to be advantageous because the discrete particles may be more easily and better dispersed in the polymer matrix. The higher surface to volume ratio of the nano-particles provides more opportunities for chemical and physical interactions with the polymer matrix.
The present invention relates to core-shell polymer particles and method for preparing them. The particles can include a crosslinked core and a crosslinked shell, where the shell is covalently connected to the core. The core-shell polymer nano-particles provide a controlled structure, size, and morphology which are useful in composite materials, such as, for example, as fillers in elastomeric polymers. Core-shell particles are widely used for rubber toughened plastics and adhesives.
Incorporation of particles as fillers in rubber compositions is common especially in the tire arts. The fillers are chosen to enhance certain characteristics of the rubber composition. The use of fillers, such as carbon black, silica, and crosslinked polymers, is well known. The addition of fillers tends to improve the physical properties of the polymer matrices to which they are added. Some specific physical properties that are improved by the addition of fillers are tensile strength and wear.
U.S. Pat. Nos. 5,395,891 and 6,127,488 disclose the use of crosslinked polybutadiene and crosslinked styrene-butadiene copolymers respectively as fillers. The purported benefits of the inclusion of these crosslinked polymer particles is lower hysteresis while the polybutadiene gels also impart improved abrasion resistance and the styrene-butadiene copolymer gels offer improve wet traction. U.S. Pat. Nos. 6,133,364, 6,207,757, and 6,242,534 disclose a rubber composition comprising a crosslinked elastomer particle and a non-crosslinked elastomer. The crosslinked elastomeric particles are functionalized to impart unique surface reactivity for improved coupling with the non-crosslinked host elastomer as well as the subsequent ability to employ coupling agents other than the conventional sulfur-bridged bis-alkoxy silanes. The benefits of the gels are reported to be lower hysteresis, improved abrasion resistance, and improved wet traction.
United States Patent Application Serial No. 2002/0007011 and United States Patent Application Serial No. 2001/0053813 to Konno et al disclose a rubber composition comprising as essential components a crosslinked polymer particle and a non-crosslinked rubber component. The rubber composition may be used to obtain a vulcanized rubber. The crosslinked polymer particles are intended for use as filler to provide a rubber composition having good processability and handling performance as well as improved tensile strength and wearing resistance when vulcanized.
Core-shell morphologies from polybutadiene-polystyrene graft copolymers are taught by U.S. Pat. Nos. 6,248,419 and 6,489,378 to Sosa et al. The teaching is directed to production of high impact polystyrene using graft copolymer particles having a polystyrene core occluded inside a polybutadiene shell and the particles have an average particle size of about 0.5 microns.
Nano-particles have also been made using seeded emulsion polymerization technique. Ferguson et al, Polymer magazine, Vol. 43, Issue 24 (November 2002), pages 6371-6382 discloses a process in which particles containing a polystyrene core may be synthesized with a poly(vinyl acetate) shell by emulsion polymerization without secondary particle nucleation in. Ferguson et al focuses on the synthesis of particles with diameters of about 400 nm and only briefly discusses the synthesis of polymer particles with diameters less than 100 nm. Ferguson et al teaches that the emulsion polymerization technique used may produce polystyrene core/poly(vinyl acetate) shell polymer particles where most of the seed particles have diameters of more than 200 nm.
The core-shell polymers of polystyrene and polybutadiene are disclosed in U.S. Pat. No. 6,777,500 to Lean et al. The shell is grafted onto the core through linkage with residual unreacted vinyl groups carried in the core. The core-shell particle can comprise a crosslinked polymer core where the crosslinked polymer core has a predetermined glass transition temperature (Tg) such that the crosslinked polymer core does not soften at service temperatures up to about 150° C.
The core-shell particle is also disclosed as a “hairy particle” as described in U.S. Provisional Patent Application Ser. No. 60/552,617, filed on Mar. 12, 2004. The hairy polymer particles can be synthesized by first making a core polymer by emulsion polymerization. After being dried the core polymer is reacted with an organo-lithium compound to make a hairless core initiator. The hairless core initiator can then be used to initiate the anionic polymerization of conjugated diolefin monomers to produce the hairy polymer particles of this invention. The final structured core-shell particle has a crosslinked core and a shell containing linear polymer brushes.
Controlled radical polymerization in emulsion polymerization is particularly attractive for synthesizing core-shell particles with controlled structures, and there is intense worldwide competition in both academic and industrial circles to develop practical emulsion processes. A review of the literature indicates that, in general, conventional emulsion polymerization techniques do not work well for controlled radical polymerization. In many cases the fundamental problems are related to slow initiation coupled with slow transport of the “active” agent or its precursor through the water phase and into the growing polymer particles. In order to circumvent these problems, many groups have used newer techniques for achieving better emulsions and faster rates. The most common technique is “miniemulsion”. With this technique, a preformed conventional emulsion of monomer(s), surfactant, a hydrophobe and water is treated under high shear conditions with a homogenizer or ultrasonic horn to prepare much finer, self-stabilized droplets. The fine droplets become the nucleus for polymerization, bypassing the need for transport through the water phase. The two main drawbacks of the miniemulsion technique are: (1) the need for specialized and expensive equipment, and (2) the use of a hydrophobe (e.g., hexadecane), which is undesirable for many potential applications.
A second technique for producing fine droplets is “microemulsion”, which typically produces initial monomer droplets in the range of 5 nm and final polymer particles in the range of 30-40 nm. This technique usually requires very large amounts of surfactant, and it rarely is used for controlled polymerization because the amount of surfactant often equals or exceeds the amount of monomer present.
A third technique for achieving controlled emulsion polymerization utilizes a seeding process to initiate polymerization. With this technique, a fraction of the monomer is first mixed with initiator, control agent, water and surfactant. This combination is mixed and allowed to react for a period of time before additional monomer is added. The intent of the first stage is to allow the initiator to form “living” oligomers or “seeds” under conditions where the surfactant-to-monomer ratio is relatively large (i.e., microemulsion). Although this technique has some advantages over miniemulsion because it does not require a hydrophobe or specialized equipment, it does not solve the fundamental problems associated with the use of controlled polymerization technologies in emulsion, such as slow initiation or long reaction times compared to solution reactions. Smulders and Monteiro, Macromolecules, 37, 4474-4483 (2004), recently disclosed a process to obtain core-shell particles using this method.
In assessing this situation, what appears to be necessary for practical emulsion processes based on controlled polymerization technology is a method for: (1) producing stable emulsions without hydrophobes or special equipment; (2) utilizing conventional surfactants and soap levels; (3) effecting rapid initiation and propagation; and (4) achieving complete conversion within a reasonable period of time. This invention pertains to a process to obtain core-shell particles or hairy particles in a practical emulsion polymerization process using controlled radical polymerization.